A function that says whether the backend instance can be instantiated in the high-level interface In general this should be true, except for some other backends (especially the tabular backends) To disable use in high-level interface, implement this function and return false.

Return the phase identification parameter (PIP) of G. Venkatarathnam and L.R. Oellrich, "Identification of the phase of a fluid using partial derivatives of pressure, volume, and temperature without reference to saturation properties: Applications in phase equilibria calculations".